1. Field of the Invention
The present invention relates to an arrangement for cleaning ventilation air coming from a spray booth for painting e.g. car bodies, and polluted with paint particles, comprising means for contacting the ventilation air with a liquid flow for separating part of the paint particles of the ventilation air, there being provided downstream of said means at least one wet electrostatic precipitator provided in an enclosure for separating the remaining particles as well as liquid droplets contained in the ventilation air and originating from said liquid flow.
2. Background Information
Venturi separators, so-called slot-type venturis, have been used for a long time for separating tacky paint particles from ventilation air coming from spray booths. In a venturi separator, the polluted ventilation air is accelerated and contacted at high velocity with a circulating flow of water which is divided into fine droplets, part of the paint particles colliding with the water droplets and being intercepted by them. See e.g. SE 196,718.
One drawback of venturi separators is that high-level noise is produced therein as the ventilation air passes through the venturi separator.
Another drawback inherent in venturi separators is that the ventilation air, in addition to residual paint particles, will contain water droplets, as well as solvents evaporated in the spray booth, after passing through the venturi separator.
In order that the objects, e.g. car bodies, which are painted in a spray booth should be provided with an acceptable surface layer, it is required, inter alia, that the ventilation air has a certain temperature and a certain relative humidity. Generally, the ventilation air is conditioned to about 23.degree. C. and a relative humidity of 60-70%. This means that much energy is required in cold climatic zones for conditioning the ventilation air, if it is not recycled to the spray booth.
Since, as pointed out above, the ventilation air having passed through the venturi separator contains both paint particles and water droplets as well as solvents evaporated in the spray booth, it must be further cleaned if it should be recycled to the spray booth. Otherwise, these remaining paint particles and water droplets would have a devastating effect on the quality of the surface layer of the objects painted in the spray booth. If the painting operation should be carried out by human beings, it is also vital to remove the above-mentioned solvent vapours from the ventilation air.
SE 8601290-3, for instance, describes a method for recycling the major portion of the ventilation air to the spray booth. This is rendered possible by conducting the ventilation air leaving the venturi separator through an electrostatic precipitator operating according to the wet separation method, before recycling the ventilation air to the spray booth. This precipitator efficiently separates the particulate pollutants of the ventilation air, that is paint particles and water droplets, whereas not the solvent vapours contained therein. It is not necessary to remove the solvent vapours from the ventilation air if painting robots are used in the spray booth.
By recycling the ventilation air according to the above-mentioned method, the energy consumption of the painting plant is reduced. However, since a conventional venturi separator is still used, this method suffers from the above-mentioned drawback of having a high sound level, and the use of a further cleaning device in the form of an electrostatic precipitator involves extra installation and running costs for the painting plant.
DE 37 05 634 describes a method of cleaning polluted ventilation air from a spray booth by directly conducting it into a conventional electrostatic precipitator operating according to the wet separation method.
According to this prior art method, the venturi separator has thus been replaced by an electrostatic precipitator, which means that no annoying sound is produced and that efficient separation of the particulate pollutants in the ventilation air can be obtained theoretically. It would however be difficult to provide sufficient room for a conventional electrostatic precipitator in the space normally occupied by the venturi separator below a spray booth.
Since the electrostatic precipitator according to the last-mentioned patent specification is disposed directly below the floor grating of the spray booth, people working in the spray booth are in danger of coming into contact with high voltage components of the precipitator. The paint should therefore be applied by robots.
Locating the electrostatic precipitator as described above also constitutes a fire and explosion hazard.